[quote="Chris Peterson"][quote="Boomer12k"]
Maybe there are still "rumblings" in this type of star.[/quote]
Very unlikely. As Art pointed out previously, neutron stars are actually quite simple in most respects, meaning that they are probably well enough understood to discount the likelihood of "rumblings" or other such activity.[/quote]
[float=right][img]http://nrumiano.free.fr/Images/Neutron_star_E.gif[/img][/float]Well... I wouldn't go that far; both crabs & neutron stars
have crusts that need 'adjustments' from time to time.
[quote=" http://en.wikipedia.org/wiki/Starquake_%28astrophysics%29#Starquake"]
<<A starquake is an astrophysical phenomenon that occurs when the crust of a neutron star undergoes a sudden adjustment, analogous to an earthquake on Earth. This is thought to be the source of the giant gamma ray flares that are produced approximately once per decade from soft gamma repeaters. Starquakes are thought to be caused by huge stresses exerted on the surface of the neutron star produced by twists in the ultra-strong interior magnetic fields.
The largest recorded starquake occurred on the ultracompact stellar corpse (magnetar) SGR 1806-20. It released gamma rays equivalent to 10[sup]36[/sup] kW in intensity. This starquake occurred 50,000 light years away; had it occurred within ten light years of Earth, it could have potentially caused a mass extinction.>>[/quote]
[clear][/clear][quote=" http://www.nasa.gov/centers/goddard/news/topstory/2006/starquake.html"]
NASA Sees Hidden Structure of Neutron Star in Starquake
Susan Hendrix, Goddard Space Flight Center 04.25.06
<<Scientists using NASA's Rossi X-ray Timing Explorer have estimated the depth of the crust on a neutron star, the densest object known in the universe. The crust, they say, is close to a mile deep and so tightly packed that a teaspoon of this material would weigh about 10 million tons on Earth. The measurement, the first of its kind, came courtesy of a massive explosion on a neutron star in December 2004. Vibrations from the explosion revealed details about the star's composition. The technique is analogous to seismology, the study of seismic waves from earthquakes and explosions that reveal the structure of the Earth's crust and interior. This new seismology technique provides a way to probe a neutron star's interior, a place of great mystery and speculation. Pressure and density are so intense here that the core might harbor exotic particles thought to have existed only at the moment of the Big Bang.
Tod Strohmayer of NASA Goddard Space Flight Center in Greenbelt, Md., presents this result in a press conference today at the April meeting of the American Physical Society in Dallas. "We think this explosion, the biggest of its kind ever observed, really jolted the star and literally started it ringing like a bell," said Strohmayer. "The vibrations created in the explosion, although faint, provide very specific clues about what makes up these bizarre objects. A neutron star's ring depends on how waves pass through layers of differing density, either slushy or solid."
A neutron star is the core remnant of a star once several times more massive than the sun. A neutron star contains about 1.4 solar masses of material crammed into a sphere only about 12 miles across. Strohmayer and Watts examined a neutron star named SGR 1806-20, about 40,000 light years from Earth in the constellation Sagittarius. The object is in a subclass of highly magnetic neutron stars called magnetars.
On December 27, 2004, the surface of SGR 1806-20 experienced an unprecedented explosion. As reported by NASA and the National Science Foundation in early 2005, this was the brightest X-ray flash ever seen from beyond our solar system. The explosion, called a hyperflare, was caused by a sudden change in the star's powerful magnetic field that cracked the crust, likely producing a massive starquake. The event was detected by several space observatories, including the Rossi Explorer, which observed the X-ray light emitted. Strohmayer and Watts think that the oscillations are evidence of global torsional vibrations within the star's crust. These vibrations, like waves moving through a rope, are analogous to the S-waves observed during terrestrial earthquakes. Their study, building on observations of vibrations from this source by GianLuca Israel of Italy's National Institute of Astrophysics, found several new frequencies during the hyperflare. Watts and Strohmayer subsequently confirmed their measurements using NASA's Ramaty High Energy Spectroscopic Solar Imager, a solar observatory that also recorded the hyperflare. And they found the evidence for a high-frequency oscillation at 625 Hz, indicative of waves traversing the crust vertically.
The abundance of frequencies---similar to a chord, as opposed to a single note---enabled the scientists to estimate the depth of the neutron star crust. This is based on a comparison of frequencies from waves traveling around the star's crust and from those traveling radially through it. The diameter of a neutron star is uncertain, but based on the estimate of about 12 miles across, the crust would be about 1 mile deep. This figure, based on the observed frequencies, is in line with theoretical estimates.
Starquake seismology holds great promise for determining many neutron star properties. Strohmayer and Watts have analyzed archived Rossi data from a dimmer 1998 magnetar hyperflare (from SGR 1900+14) and found telltale oscillations here, too, although not strong enough to determine the crust thickness. A larger neutron star explosion detected in X-rays might reveal deeper secrets, such as the nature of matter at the star's core. One exciting possibility is that the core might contain free quarks. Quarks are the building blocks of protons and neutrons, and under normal conditions are always tightly bound together. Finding evidence for free quarks would aid in understanding the true nature of matter and energy. Laboratories on Earth, including massive particle accelerators, cannot generate the energies needed to reveal free quarks.
"Neutron stars are great laboratories for the study of extreme physics," said Watts. "We'd love to be able to crack one open, but since that's probably not going to happen, observing the effects of a magnetar hyperflare on a neutron star is perhaps the next best thing.">>[/quote]
[quote=" http://nrumiano.free.fr/Estars/neutrons.html"]
<<Following the explosion of a supernova, a neutron star is created with a temperature probably over 1000 billion degrees. It will rapidly cool in less than 1000 years, to 1 million degrees. After that, its temperature will decrease much more slowly. At its birth, this neutron star is recovering the rotation of the previous star, following the conservation of angular momentum . It will rotate at a very high speed. The Crab pulsar inside the nebula, for example, spins 30 times a second.
Until recently, one supposed that a neutron star began by rotating at a very high speed, and slowed down with time. This scenario seems satisfactory for a lone star, but in the case of a binary system, where the companion is a small sized star, magnetic coupling effects with the forming accretion disk seems to cause a later acceleration of the spinning speed.
The powerful magnetic and electrical field which surrounds the star will generate a thin beam of light, in the radio-wave frequencies. This beam sweeps across the sky, in the same way a lighthouse beam sweeps across the sea. These stars are called pulsars. Because the magnetic axis of the pulsar is not aligned with its rotational axis, the radio emission, generated by particles trapped in the magnetic field lines, will sweep across the sky, like a lighthouse beam sweeps across the sea. Some pulsars rotate a few hundred times per second. The rotation of a pulsar is exceedingly precise, and can be used as a cosmic clock. In particular, the system known as PSR 1913+16, made up of two pulsars, allowed the scientists to measure the very small effects of gravitational waves, predicted by general relativity.
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Magnetars
Neutron stars exhibit a very powerful magnetic field, anchored to their surface. This field comes from the magnetic field of the initial star, compressed by the collapse of the supernova. It is about 10[sup]12[/sup] Gauss, i.e. a trillion times more powerful than the Earth's magnetic field. The core of a neutron star is an electrical conductor, because it contains a trace of free electrons and protons. If the star is born rotating fast enough (at least 200 rev/second), the liquid core is able to start a "dynamo action" during the first 20 seconds or so, which is enough to enhance the magnetic field eight hundred to one thousand times (about 8x10[sup]14[/sup] Gauss).
The magnetic field rotates with the star, being anchored to its surface. With such a strength, the magnetic waves and the related, magnetically-powered charged particles will carry off the star's rotational energy in a very efficient way, and suddenly brake its spinning movement. In some thousand years, the spinning velocity of the star will become as low as one revolution every five to ten seconds. Such a star is called a magnetar (contraction of magnetic star). When a magnetic field is this powerful, it can move material in the star's interior, and so apply very high stresses over the solid crust ; sometimes this crust can break, in the same way that the Earth's crust breaks in an earthquake under the stresses. At this time, the star ejects bursts of highly energetic particles, which will produce a brief, but intense emission of hard X-rays (the neutron star radiates at this moment as much energy as the Sun radiates in 1000 years).
This radiation emission can repeat sporadically. This phenomenon is called : Soft Gamma Repeaters. Some magnetars radiate X-rays with a period of about ten seconds : they are called AXP : Anomalous X-ray Pulsars. This radiation comes from hot matter trapped inside the lines of the magnetic field, rotating with the star. After 10,000 years, the magnetic energy source in these stars begins to run down, and the magnetar slowly becomes invisible. It is possible that 10% of neutron stars are magnetars.
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Strange stars
[float=right][img3="Strange stars in the night?"]http://nrumiano.free.fr/Images/int_neutron_E.jpg[/img3][/float]
If the mass of the neutron star is high enough, the density in its core can be so high that the appearance of inside a neutron starheavy particles could become possible : hyperons, pions ... In fact, nobody really knows what can happen, because the theory about the strong interaction (which rules the environment) under high density is not currently understood well enough.
An hypothesis imagined in 1984 by the physicist Edward Witten would be the quark deconfinement, with the outbreak of a plasma of quarks u and d and gluons, under the effect of an external excitation - an high energy cosmic ray for example. This plasma is unstable and these quarks can desintegrate, giving now quarks s (strange). This core of quarks s and gluons is going to convert progressively the remaining neutrons of the star, to finally end in a total transformation (with a possible exception of a fine crust) in strange matter - so called, because mainly made up of quarks s . This very fast transformation, between one second and no more than ten minutes, leads to a "quark star", also called "strange star".
Such a star is not ruled only by gravitation, but essentially by quantum chromodynamics (QCD). So, it has no minimal mass, and a radius proportional to its mass. A strange star would typically have a mass between one and two solar masses, and a radius about 10km, less than the radius of a neutron star.
The fact that the star has a crust or no leads to great consequences for observations : if the star does not have a crust, it will not emit any visible light. Conversely, if it has a crust of nuclear matter, its surface properties will be the same as a neutron star, and could for example to behave as a pulsar - a very fast pulsar because the radius of the star is less and the quark-gluon plasma is more viscous than the liquid of neutrons.
Theoretical models forecast a faster cooling for a quark star than for a neutron star, but this could be questioned if a superfluidity phenomenon arises in the neutron star, drastically lowering its caloric value.
In 1996, the ROSAT satellite discovered an X-ray source, called RX J1856.6-3754 at about 450 light years of the Earth. Later measurements by the Chandra observatory seem to indicate a diameter of about 10 km for the star, too little for a neutron star. Obviously, this measurement is liable to doubt, the accuracy of measurement of such a little diameter at such a distance being very relative. Other similar neutron stars have been discovered since this one, but the measurement -indirect, of course- of their sizes is always very difficult and subject to many errors.
At last, some astrophysicists are doubting about the very existence of strange stars, not on a theoretical point of view, but because of the lack of a simple and realistic scenario to create them. The conditions to create a black hole, as we are about to discover, are much easier to fulfill than the ones necessary to create a strange star.>>[/quote]